Exhaust treatment system with an oxidation device for NO2 control

ABSTRACT

An exhaust treatment system includes an oxidation device having a plurality of channels through which a flow of exhaust flows. A first portion of the channels of the oxidation device are coated with catalytic material for converting NO to NO 2  and a second portion of the channels are uncoated with the catalytic material. The exhaust treatment system also includes a selective catalytic reduction device disposed downstream from the oxidation device. The selective catalytic reduction device is configured to receive the flow of exhaust passing through the oxidation device.

TECHNICAL FIELD

The present disclosure relates generally to an exhaust treatment system,and more particularly, to an exhaust treatment system with NO₂ control.

BACKGROUND

Internal combustion engines, including diesel engines, gasoline engines,gaseous fuel-powered engines, and other engines known in the art, mayexhaust a complex mixture of air pollutants. The air pollutants may becomposed of gaseous and solid compounds, including particulate matter,nitrogen oxides (NOx), and sulfur compounds. Due to heightenedenvironmental concerns, exhaust emission standards have becomeincreasingly stringent. The amount of pollutants emitted from an enginemay be regulated depending on the type, size, and/or class of engine.One method that has been implemented by engine manufacturers to complywith the regulation of NOx exhausted to the environment has been toimplement a strategy called selective catalytic reduction (SCR).

SCR is a process by which gaseous or liquid reductant (e.g., urea orammonia) is added to the flow of exhaust from an engine. The combinedflow is then absorbed onto a catalyst. The reductant reacts with NOx inthe flow of exhaust to form H₂O and N₂. SCR may be more effective when aratio of NO to NO₂ in the flow of exhaust supplied to the SCR catalystis about 50:50. Some engines, however, may produce a flow of exhausthaving a NO to NO₂ ratio of approximately 95:5. In order to increase therelative amount of NO₂ to achieve a NO to NO₂ ratio of closer 50:50, adiesel oxidation catalyst (DOC) may be located upstream of the SCRcatalyst to convert NO to NO₂.

One system that includes a DOC to increase a relative amount of NO₂ in aflow of exhaust is described in U.S. Pat. No. 6,846,464 (the '464patent) issued to Montreuil et al. The '464 patent describes a catalyticdevice including two chambers. The first chamber includes tubes coatedwith a catalytic material such as platinum that oxidizes NO andhydrocarbons. The second chamber includes tubes coated with a catalyticmaterial such as palladium that oxidizes NO and hydrocarbons. An SCRcatalyst is provided downstream from the two chambers of the catalyticdevice.

Although the system of the '464 patent may provide an oxidation catalystthat increases the amount of NO₂ in the flow of exhaust, all of thetubes of the oxidation catalyst are coated with an NO oxidizingmaterial, such as platinum or palladium. Therefore, the entire flow ofexhaust contacts either the platinum or the palladium coating on theoxidation catalyst. As a result, there is a risk of providing too muchNO₂ compared to NO. When there is too much NO₂, NOx reduction in thereduction catalyst is much slower, and therefore, a larger reductioncatalyst is necessary to effectively reduce NOx in the flow of exhaust.

The disclosed system is directed to overcoming one or more of theproblems set forth above.

SUMMARY OF THE INVENTION

In one aspect, the present disclosure is directed to an exhausttreatment system. The exhaust treatment system includes an oxidationdevice having a plurality of channels through which a flow of exhaustflows. A first portion of the channels of the oxidation device arecoated with catalytic material for converting NO to NO₂ and a secondportion of the channels are uncoated with the catalytic material. Theexhaust treatment system also includes a selective catalytic reductiondevice disposed downstream from the oxidation device. The selectivecatalytic reduction device is configured to receive the flow of exhaustpassing through the oxidation device.

In another aspect, the present disclosure is directed to a method fortreating a flow of exhaust. The method includes generating the flow ofexhaust and passing the flow of exhaust through a plurality of channelsof an oxidation device. A first portion of the channels of the oxidationdevice is coated with catalytic material for converting NO to NO₂, and asecond portion of the channels is uncoated with the catalytic material.The method also includes directing the combined flow of exhaust to adownstream selective catalytic reduction device disposed downstream fromthe oxidation device.

In yet another aspect, the present disclosure is directed to an exhausttreatment system. The exhaust treatment system includes an upstreaminjector configured to inject reductant into a flow of exhaust and anupstream selective catalytic reduction device disposed downstream fromthe upstream injector. The upstream selective catalytic reduction deviceis configured to receive the flow of exhaust. The exhaust treatmentsystem also includes an oxidation device disposed downstream from theupstream selective catalytic reduction device. The oxidation device hasa plurality of channels through which the flow of exhaust flows. A firstportion of the channels is coated with catalytic material for convertingNO to NO₂, and a second portion of the channels is uncoated with thecatalytic material. The exhaust treatment system further includes adownstream selective catalytic reduction device disposed downstream fromthe oxidation device. The downstream selective catalytic reductiondevice is configured to receive the flow of exhaust.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary disclosed engineand exhaust treatment system;

FIG. 2 is a cross-sectional view of an exemplary disclosed oxidationdevice for the exhaust treatment system of FIG. 1;

FIG. 3 is a diagrammatic illustration of an exemplary disclosed engineand exhaust treatment system having two legs;

FIGS. 4A-4D are diagrammatic illustrations of exemplary disclosed firstlegs of the exhaust treatment system of FIG. 3;

FIG. 5 is a diagrammatic illustration of another exemplary disclosedengine and exhaust treatment system having two legs;

FIG. 6 is a diagrammatic illustration of another exemplary disclosedengine and exhaust treatment system;

FIG. 7 is a diagrammatic illustration of yet another exemplary disclosedengine and exhaust treatment system having two legs;

FIG. 8 is a diagrammatic illustration of a further exemplary disclosedengine and exhaust treatment system having two legs;

FIGS. 9A and 9B are diagrammatic illustrations of exemplary disclosedfirst legs of the exhaust treatment system of FIG. 8; and

FIG. 9C is a diagrammatic illustration of an exemplary disclosed secondleg of the exhaust treatment system of FIG. 8.

Detailed Description

Reference will now be made in detail to exemplary embodiments, which areillustrated in the accompanying drawings. Wherever possible, the samereference numbers will be used throughout the drawings to refer to thesame or like parts.

As shown in FIG. 1, a power source, such as an engine 10, of a machineis provided. The disclosed embodiment may be applicable to various typesof machines such as, for example, a fixed or mobile machine thatperforms some type of operation associated with an industry such asmining, construction, farming, transportation, power generation, treeharvesting, forestry, or any other industry known in the art. The engine10 may be an internal combustion engine, such as, for example, a dieselengine, a gasoline engine, a gaseous fuel-powered engine, or any otherengine apparent to one skilled in the art. The engine 10 mayalternatively be another source of power such as a furnace or any othersuitable source of power for a powered system such as a factory or powerplant. Operation of the engine 10 may produce power and a flow ofexhaust. For example, each combustion chamber (not shown) of the engine10 may mix fuel with air and combust the mixture therein to produce aflow of exhaust directed into an exhaust passageway. The flow of exhaustmay contain carbon monoxide, NOx, carbon dioxide, aldehydes, soot,oxygen, nitrogen, water vapor, and/or hydrocarbons such as hydrogen andmethane.

An exhaust treatment system 20 is provided with the engine 10. The flowof exhaust may be fluidly communicated from the engine 10 to the exhausttreatment system 20. Although not shown, other components such as, forexample, one or more turbochargers, or any other component for treatingor handling exhaust known in the art may be disposed between the exhaustpassageway of the engine 10 and the inlet of the exhaust treatmentsystem 20. In addition, other emission control devices, e.g., exhaustgas recirculation devices, may be disposed within or fluidly connectedto the exhaust passageway of the engine 10.

The exhaust treatment system 20 shown in FIG. 1 may optionally includean upstream injector 21 disposed upstream from an upstream SCR device23. The upstream injector 21 may be provided to inject reductant, suchas urea, ammonia, and/or other elements or compounds capable ofchemically reducing, e.g., NOx, contained within the flow of exhaust atpredetermined timings, pressures, and flow rates.

An upstream SCR device 23, such as an SCR catalyst, may be disposeddownstream of the upstream injector 21. The upstream SCR device 23 maychemically reduce the amount of NOx in the flow of exhaust. Reductantinjected into the flow of exhaust upstream from the upstream SCR device23 may be absorbed onto the upstream SCR device 23 so that the reductantmay react with NOx in the flow of exhaust to form H₂O (water vapor) andN₂ (nitrogen gas).

The exhaust treatment system 20 shown in FIG. 1 includes an oxidationdevice 22, such as a DOC, that receives the flow of exhaust directlyfrom the engine 10 or from the upstream SCR device 23, if provided. Ifthe upstream SCR device 23 is provided, it may be close-coupled to theoxidation device 22. The oxidation device 22 may be a device with aporous ceramic honeycomb-like or metal mesh structure. FIG. 2 shows across-sectional view of an exemplary honeycomb-like structure of theoxidation device 22. The oxidation device 22 includes holes 22 a,channels, passageways, or other openings through which the flow ofexhaust may pass. According to an embodiment, the oxidation device 22may be “partially-loaded,” i.e., a percentage less than 100% (e.g.,approximately 50%) of the holes may be coated with platinum or anothermaterial for oxidizing NO, such as palladium, metal oxide, rhodium, orother precious metal. Alternatively, the percentage of holes 22 a coatedwith the NO oxidizing material may be, e.g., approximately 25%, 35%,45%, 55%, 65%, 75%, etc., or any percentage therebetween. The remainingholes may be uncoated. The percentage of holes 22 a coated may bedetermined experimentally as described below based on the application toobtain a target amount of NO₂ in the resulting flow of exhaust, e.g.,50% NO and 50% NO₂. Accordingly, only a percentage of the total flow ofexhaust flowing through the oxidizing device 22 contacts the NOoxidizing material, and the percentage of the total flow of exhaustcontacting NO oxidizing material depends on the percentage of holes 22 acoated.

As shown in FIG. 1, the exhaust treatment system 20 may include aparticulate filter 24, which may be disposed downstream of the oxidationdevice 22. The particulate filter 24 may be a non-catalyzed filter andmay include a wire mesh or ceramic honeycomb filtration media utilizedto remove particulate matter from the flow of exhaust. Alternatively,the particulate filter 24 may be another type of device that physicallycaptures particulates, ash, or other materials from the flow of exhaust.The particulate filter 24 may be a wall flow type filter, flow throughtype filter, or other type of filter known in the art. A “wall flowtype” filter may refer to, e.g., a filter that includes a plurality ofpassages and opposite ends of adjacent passages may be blocked orplugged in order to force the flow of exhaust to travel radially througha plurality of relatively thin porous walls. A “flow through type”filter may refer to, e.g., a filter that can capture and storeparticulate matter while allowing open passages through which the flowof exhaust may flow. For example, a flow through type filter may includesheets or other blocking mechanisms that may divert particulate mattertoward a mesh lining sides of the open passages. Alternatively, theparticulate filter 24 may be omitted if the flow of exhaust from theengine 10 has lower amounts of particulate matter.

The particulate filter 24 may then be connected to a heat source 25. Theheat source 25 may be any conventional heat source known in the art.Such heat sources may include, e.g., a furnace, an electric heater, afuel burner, etc. The heat source 25 may direct heat to any particulatefilter located in the exhaust treatment system 20, such as theparticulate filter 24, so as to thermally age the particulate matterdeposited in the particulate filter. Alternatively, the heat source 25may be omitted, and the engine 10 may heat the flow of exhaust, whichmay heat the particulate matter in the particulate filter 24. The heatsource 25 may be disposed upstream from the particulate filter 24 anddownstream from the engine 10. In the exemplary embodiment shown in FIG.1, the heat source 25 is disposed upstream from the upstream injector 21and the upstream SCR device 23, and downstream from the engine 10.

An injector 26, such as an injector described above in connection withthe upstream injector 21, may be provided to inject reductant, such asurea or ammonia, into the flow of exhaust downstream from theparticulate filter 24. A controller 12 may receive input viacommunication lines 14 from a variety of sources including, for example,sensors configured to measure temperature, speed, fuel quantityconsumed, and/or other operating characteristics of the engine 10. Forexample, the timing of the injections by one or more of the injectors21, 26 may be synchronized with sensory input received from a sensor 40(FIGS. 4C, 4D, 6, and 8), such as a temperature sensor as describedbelow, a NOx sensor, a flow sensor, a pressure sensor, a timer, or anyother similar sensory device. It is further contemplated that injectionsmay occur on a set periodic basis, in addition to or regardless ofpressure or temperature conditions, if desired. In order to accomplishthese specific injection events, the controller 12 may control operationof one or more of the injectors 21, 26 in response to the one or moreinputs.

The controller 12 may use these inputs to form a control signal based ona pre-set control algorithm. The control signal may be transmitted fromthe controller 12 via the communication lines 14 to various actuationdevices, such as the injectors 21, 26. The controller 12 may embody asingle microprocessor or multiple microprocessors that include a meansfor controlling an operation of the injectors 21, 26. Numerouscommercially available microprocessors can be configured to perform thefunctions of the controller 12. The controller 12 may include componentsrequired to run an application such as, for example, a memory, asecondary storage device, and a processor, such as a central processingunit or any other means known in the art. It is contemplated that thecontroller may further be communicatively coupled with one or morecomponents of the engine 10 to change the operation thereof. Thus, theengine 10 and the injectors 21, 26 may be connected to the controller12, and optionally, the controller 12 may be integrated into the engine10.

Alternatively, the sensor 40 may embody both a physical sensor and avirtual sensor, which is included in the controller 12, that generates asignal based on a map-driven estimate. The physical sensor may detectand communicate to the controller 12 parameters, e.g., one or more ofengine fuel/air settings, engine operating speed, engine load, enginefuel injection profile, other engine operating parameters, exhausttemperature, exhaust flow rate, a temperature of any element within theexhaust treatment system 20, etc. The virtual sensor may evaluate thesignals received from one or more physical sensors, and, usingrelationships contained within one or more maps stored in a memory ofthe controller 12, may estimate an operating parameter, e.g., theexpected exhaust gas NO:NO₂ ratio, based on the sensed parameters.Alternatively, the sensor 40 may be a physical sensor that is capable ofsensing the NO:NO₂ ratio, an amount of NOx, etc.

An SCR device 28, such as an SCR catalyst or other type of SCR devicedescribed above in connection with the upstream SCR device 23, may bedisposed downstream of the particulate filter 24 and the injector 26.Urea injected by the injector 26 may decompose to ammonia, and the SCRdevice 28 may facilitate a reaction between the ammonia and NOx in theflow of exhaust to produce water and nitrogen gas, thereby removing NOxfrom the flow of exhaust. After exiting the SCR device 28, the flow ofexhaust may be output from the exhaust treatment system 20, e.g.,released into the surrounding atmosphere. Alternatively, a second SCRdevice 28 a may be disposed downstream from the SCR device 28 to receivethe flow of exhaust before it is output from the exhaust treatmentsystem 20. The second SCR device 28 a may further reduce the amount ofNOx in the flow of exhaust, e.g., if the volume of exhaust flow ishigher.

FIG. 3 illustrates another exemplary embodiment of an exhaust treatmentsystem 30. As shown in FIG. 3, the oxidation device 22 of the exhausttreatment system 20 shown in FIG. 1 may be replaced with a dual-legsubsystem having a first leg 30 a and a second leg 30 b. The SCR device28 described above may be located downstream from the two legs 30 a, 30b. The first leg 30 a includes a catalyzed particulate filter 32, suchas a catalyzed diesel particulate filter (CDPF). The catalyzedparticulate filter 32 may be, e.g., a particulate filter, such as theparticulate filters described above in connection with the particulatefilter 24 (e.g., a wall flow type filter, a flow through type filter, orother type of filter) having a sponge-like or other type of porous orfoam-like material that may be coated uniformly with platinum or anothermaterial for oxidizing NO, such as palladium, metal oxide, rhodium, orother precious metal. Accordingly, either a substantial portion of theexhaust or all of the exhaust flowing through the catalyzed particulatefilter 32 contacts the NO oxidizing material. In the catalyzedparticulate filter 32, the NO oxidizing material oxidizes NO to formNO₂. The NO₂ may react with carbon (soot) in the particulate matter toform CO and NO. As a result, the amount of carbon (soot) in thecatalyzed particulate filter 32 may be reduced, thereby regenerating thecatalyzed particulate filter 32.

The second leg 30 b may include, in order from upstream to downstream,the oxidation device 22 and the particulate filter 24 described above inconnection with the embodiment of FIG. 1. The oxidation device 22 andthe particulate filter 24 in the second leg 30 b may be provided in asingle can or housing. Optionally, a valve 34 may be provided downstreamfrom the particulate filter 24 in the second leg 30 b. The valve 34 maycontrol the amount of exhaust flowing through the second leg 30 b andtherefore may control the allocation of flow between the two legs 30 a,30 b. For example, one or more sensors 40 described above in connectionwith the embodiment of FIG. 4C may be provided for monitoring the NO:NO₂ratio of the flow of exhaust in the first leg 30 a and/or the second leg30 b, e.g., downstream from the catalyzed particulate filter 32 and/orthe particulate filter 24. Alternatively, the valve 34 may be omitted,and the catalyzed particulate filter 32 in the first leg 30 a may besized with respect to the oxidation device 22 and the particulate filter24 in the second leg 30 b to control the allocation of flow between thetwo legs 30 a, 30 b. For example, the catalyzed particulate filter 32 inthe first leg 30 a may be sized so that approximately 50% of the totalflow of exhaust is directed to the catalyzed particulate filter 32.Alternatively, the catalyzed particulate filter 32 may be sized so thatapproximately 45%, 55%, 60%, etc., or any percentage therebetween of theflow of exhaust is directed to the catalyzed particulate filter 32. Theremaining exhaust flows to the second leg 30 b. The SCR device 28 andthe second SCR device 28 a, described above in connection with theembodiment of FIG. 1, may be located downstream from the two legs 30 a,30 b. Alternatively, the second SCR device 28 a may be omitted.

FIGS. 4A-4D illustrate alternative exemplary embodiments of the firstleg 30 a of the exhaust treatment system 30 of FIG. 3. In a first leg 30a′ shown in FIG. 4A, the catalyzed particulate filter 32 of the firstleg 30 a shown in FIG. 3 may be replaced by, in order from upstream todownstream, the oxidation device 22 and the particulate filter 24described above in connection with the embodiment of FIG. 1.Alternatively, in a first leg 30 a″ shown in FIG. 4B, the oxidationdevice 22, described above in connection with the embodiment of FIG. 1,may be disposed upstream of the catalyzed particulate filter 32 of thefirst leg 30 a shown in FIG. 3. As another alternative, in a first leg30 a′″ shown in FIG. 4C, the upstream injector 21, described above inconnection with the embodiment of FIG. 1, may be disposed upstream ofthe catalyzed particulate filter 32 of the first leg 30 a shown in FIG.3. Alternatively, the upstream injector 21 may be disposed upstream ofthe first and second legs 30 a′″, 30 b and downstream of the heat source25. The sensor 40 may be provided for monitoring a temperature of thecatalyzed particulate filter 32. Signals indicating the sensedtemperature are transmitted from the sensor 40 to the controller 12 viathe communication line 14. As a further alternative, in a first leg 30a′″ shown in FIG. 4D, the upstream injector 21 and the upstream SCRdevice 23, described above in connection with the embodiment of FIG. 1,may be disposed upstream of the catalyzed particulate filter 32 of thefirst leg 30 a shown in FIG. 3. The upstream SCR device 23 may beclose-coupled to the catalyzed particulate filter 32. The sensor 40described above in connection with the embodiment of FIG. 4C may beprovided for monitoring the temperature of the catalyzed particulatefilter 32.

FIG. 5 illustrates a further exemplary embodiment of an exhausttreatment system 50 in which the dual-leg subsystem of the exhausttreatment system 30 shown in FIG. 3 may be replaced with a dual-legsubsystem having a first leg 50 a and a second leg 50 b. The first leg50 a may include, in order from upstream to downstream, the catalyzedparticulate filter 32 and the valve 34, described above in connectionwith the embodiment of FIG. 3. The second leg 50 b may include theparticulate filter 24 and the valve 34 described above in connectionwith the embodiments of FIGS. 1 and 3. The SCR device 28 and the secondSCR device 28 a, described above in connection with the embodiment ofFIG. 1, may be located downstream from the two legs 50 a, 50 b.Alternatively, the second SCR device 28 a may be omitted. As anotheralternative, for example, when a lesser reduction of particulate matterfrom the flow of exhaust is necessary, the particulate filter 24 in thesecond leg 50 b may be omitted. As yet another alternative, for example,when the flow of exhaust from the engine 10 has a low amount ofparticulate matter, the catalyzed particulate filter 32 in the first leg50 a may be replaced by an oxidization device, such as a DOC or othertype of oxidation device described above in connection with theoxidation device 22 of FIG. 1, and the particulate filter 24 in thesecond leg 50 b may be omitted.

FIG. 6 illustrates yet another exemplary embodiment of an exhausttreatment system 60. As shown in FIG. 6, the exhaust treatment system 20shown in FIG. 1 may further include an upstream oxidation device 62,such as a DOC or other type of oxidation device described above inconnection with the oxidation device 22 of FIG. 1. Downstream from theupstream oxidation device 62, the exhaust treatment system 60 mayfurther include, in order from upstream to downstream, the upstreaminjector 21, the upstream SCR device 23, the oxidation device 22, theparticulate filter 24, the injector 26, and the SCR device 28 describedabove in connection with the embodiment of FIG. 1. Alternatively, theparticulate filter 24 may be omitted. The upstream oxidation device 62and the oxidation device 22 may be similar, but may have differentpercentages of the holes 22 a (FIG. 2) coated with the NO oxidizingmaterial. For example, the upstream oxidation device 62 may haveapproximately 25% of the holes 22 a coated, and the upstream oxidationdevice 72 in the second leg 70 b may have approximately 65% to 100% ofthe holes 22 a coated. The sensor 40 described above in connection withthe embodiment of FIG. 4C may be provided for monitoring the temperatureand/or the NO:NO₂ ratio of the flow of exhaust downstream from theparticulate filter 24. Alternatively, the sensor 40 may be providedanywhere downstream from the engine 10, e.g., between the oxidationdevice 22 and the SCR device 28.

FIG. 7 illustrates a further exemplary embodiment of an exhausttreatment system 70. As shown in FIG. 7, the exhaust treatment system 70may include a dual-leg subsystem having a first leg 70 a and a secondleg 70 b. The first leg 70 a includes, in order from upstream todownstream, the upstream oxidation device 62, the upstream injector 21,the upstream SCR device 23, and the particulate filter 24 describedabove in connection with the embodiments of FIGS. 1 and 6. The secondleg 70 b also includes, in order from upstream to downstream, theupstream injector 21, the upstream SCR device 23, and the particulatefilter 24, which are identical or similar to the like elements in thefirst leg 70 a. An upstream oxidation device 72 is located upstream fromthe upstream injector 21 in the second leg 70 b. The upstream oxidationdevice 62 in the first leg 70 a and the upstream oxidation device 72 inthe second leg 70 b may be similar, but may have different percentagesof the holes 22 a (FIG. 2) coated with the NO oxidizing material. Forexample, the upstream oxidation device 62 in the first leg 70 a may haveapproximately 25% of the holes 22 a coated, and the upstream oxidationdevice 72 in the second leg 70 b may have approximately 65% to 100% ofthe holes 22 a coated. The SCR device 28, described above in connectionwith the embodiment of FIG. 1, may be located downstream from the twolegs 70 a, 70 b.

FIG. 8 illustrates a further exemplary embodiment of an exhausttreatment system 80. As shown in FIG. 8, the exhaust treatment system 80may include a dual-leg subsystem having a first leg 80 a and a secondleg 80 b. The first leg 80 a may be similar to the first leg 30 a″″ ofthe exhaust treatment system 30 shown in FIGS. 3 and 4D, i.e., the firstleg 80 a may include, from upstream to downstream, the upstream injector21, the upstream SCR device 23, and the catalyzed particulate filter 32connected to the sensor 40. The second leg 80 b may be similar to thesecond leg 70 b of the exhaust treatment system 70 shown in FIG. 7,i.e., the second leg 80 b may include, from upstream to downstream, theupstream oxidation device 72, the upstream injector 21, the upstream SCRdevice 23, and the particulate filter 24. The injector 26 and the SCRdevice 28 described above may be located downstream from the two legs 80a, 80 b.

FIGS. 9A and 9B illustrate alternative exemplary embodiments of thefirst leg 80 a of the exhaust treatment system 80 of FIG. 8. As shown inFIG. 9A, a first leg 80 a′ may include, from upstream to downstream, theupstream injector 21, the upstream SCR device 23, the oxidation device22, and the particulate filter 24 as provided in the embodiment ofFIG. 1. As shown in FIG. 9B, a first leg 80 a″ may include, fromupstream to downstream, the upstream oxidation device 62, the upstreaminjector 21, the upstream SCR device 23, and the particulate filter 24as described above in connection with the embodiment of FIG. 7. FIG. 9Cillustrates an alternative exemplary embodiment of the second leg 80 bof the exhaust treatment system 80 of FIG. 8. As shown in FIG. 9C, asecond leg 80 b′ may include, from upstream to downstream, the upstreaminjector 21, the upstream SCR device 23, the oxidation device 22, andthe particulate filter 24 as provided in the embodiment of FIG. 1.

INDUSTRIAL APPLICABILITY

The disclosed exhaust treatment system may be provided in any machine orpowered system that includes a power source producing a flow of exhaust,such as an engine. The disclosed exhaust treatment system may increasethe amount of NO₂ relative to NO in the flow of exhaust upstream of theSCR device so that the SCR device can more quickly and efficientlyreduce the amount of NOx. The operation of the exhaust treatment systemwill now be explained.

According to the embodiment of the exhaust treatment system 20 shown inFIG. 1, the flow of exhaust from the engine 10 may be heated by the heatsource 25 before being directed to the oxidation device 22. Optionally,e.g., during cold start conditions, reductant may be injected into theflow of exhaust by the upstream injector 21 and then the flow ofreductant and exhaust may be directed to the upstream SCR device 23 toreduce the amount of NOx in the flow of exhaust before being directed tothe oxidation device 22. Then, the flow of exhaust may be directed tothe particulate filter 24 where particulate matter may be removed. Afterexiting the particulate filter 24, reductant is injected into the flowof exhaust by the injector 26, and the flow of exhaust is directed tothe SCR device 28 and optionally the second SCR device 28 a, whichreduce the amount of NOx in the flow of exhaust.

Efficiency of the NOx reduction by the SCR device 28 may be at leastpartially dependent on the ratio of NO₂ to NOx in the flow of exhaust.In particular, NOx reduction by the SCR device 28 may be faster and moreefficient when the ratio of NO₂ to NOx in the flow of exhaust isapproximately 50:50. According to an exemplary embodiment, the oxidationdevice 22 may convert some of the NO in the flow of exhaust to NO₂ sothat the NO:NO₂ ratio is closer to 50:50. For example, approximately 50%to 75% of the holes 22 a of the oxidation device 22 may be coated withplatinum. In one embodiment, approximately 75% of the holes 22 a of theoxidation device 22 may be coated with platinum. As a result, theexhaust treatment system 20 shown in FIG. 1 may reduce particulatematter and may provide a greater reduction in NOx before releasing theflow of exhaust to the surrounding atmosphere.

The oxidation device 22 also allows the regeneration of the particulatefilter 24. The oxidation device 22 increases the amount of NO₂ such thatNO₂ reacts with carbon (soot) in the particulate matter to form CO andNO. As a result, the amount of carbon (soot) in the particulate filter24 is reduced, thereby regenerating the particulate filter 24 andreducing the risk of having the particulate matter build up and clog theparticulate filter 24.

According to the embodiment of the exhaust treatment system 30 shown inFIG. 3, the flow of exhaust from the engine 10 may be heated by the heatsource 25 before being directed to the first and second legs 30 a, 30 b.A first portion of the exhaust flows through the first leg 30 a, whereall or a substantial amount of the first portion of the exhaust contactsthe platinum coating of the catalyzed particulate filter 32 to convertsome of the NO to NO₂. The increase in NO₂ may allow the catalyzedparticulate filter 32 to regenerate as described above and may providean increased amount of NO₂ from the first leg 30 a to the SCR device 28.A second portion of the exhaust flows through the second leg 30 b, wherethe second portion of the exhaust is directed to the partially-loadedoxidation device 22. In an exemplary embodiment, the oxidation device 22has a percentage less than 50%, e.g., approximately 25%, of the holes 22a coated with platinum. Accordingly, the oxidation device 22 may convertsome of the NO to NO₂ in the second portion of the exhaust. The increasein NO₂ may allow the regeneration of the particulate filter 24 in thesecond leg 30 b, as described above in connection with the oxidationdevice 22 and the particulate filter 24 of the embodiment shown inFIG. 1. The increase in NO₂ may also allow the SCR device 28 to performNOx reduction faster and more efficiently as described above. Afterexiting the oxidation device 22, the second portion of the exhaust maybe directed to the particulate filter 24 where particulate matter may beremoved.

The respective portions of exhaust from the first and second legs 30 a,30 b are combined, and reductant is injected by the injector 26 into thecombined flow. Then, the combined flow is directed to the SCR device 28,which reduces the amount of NOx in the combined flow. According to theembodiments shown in FIGS. 3 and 4A-4D, the components in the first leg30 a, 30 a′, 30 a″, 30 a′″, 30 a″″ may be sized relative to theoxidation device 22 and the particulate filter 24 in the second leg 30 bsuch that the combined flow from the first leg 30 a, 30 a′, 30 a″, 30a′″, 30 a″″ and the second leg 30 b has a NO:NO₂ ratio that is closer to50:50. Alternatively or in addition, the sensor 40 and/or the valve 34may be provided in the second leg 30 b to also control the respectiveamounts of flow in the two legs 30 a, 30 a′, 30 a″, 30 a′″, 30 a″″ and30 b. As a result, the reduction of NOx in the SCR device 28 may be moreefficient and faster. The exhaust treatment system 30 may reduceparticulate matter (with the components in the first leg 30 a, 30 a′, 30a″, 30 a′″, 30 a″″ and the particulate filter 24 in the second leg 30b), and may provide a greater reduction in NOx before releasing the flowof exhaust to the surrounding atmosphere.

Alternatively, one or more of the sensors 40 may be provided, e.g., inone or both of the two legs 30 a, 30 a′, 30 a″, 30 a′″, 30 a″″ and 30 bor between the two legs and the SCR device 28. Accordingly, thecontroller 12 may adjust the allocation of flow between the two legs 30a, 30 a′, 30 a″, 30 a′″, 30 a″″ and 30 b, e.g., by controlling the valve34, based on a sensed condition (e.g., the NO:NO₂ ratio) to provide moreaccurate control of the amount of NO₂ supplied to the SCR device 28.Closed loop control of the ratio of NO:NO₂ in the exhaust may beachieved.

According to the embodiment shown in FIG. 4A, the first portion of theexhaust may flow through the first leg 30 a′, where the first portion ofthe exhaust may be directed to the partially-loaded oxidation device 22and then the particulate filter 24. In the exemplary embodiment, theoxidation device 22 may have approximately 50% or a lower percentage ofthe holes 22 a coated with platinum. Accordingly, the oxidation device22 may convert some of the NO to NO₂ in the first portion of theexhaust. The increase in NO₂ may allow the regeneration of theparticulate filter 24 in the first leg 30 a′, as described above inconnection with the oxidation device 22 and the particulate filter 24 ofthe embodiment shown in FIG. 1. The increase in NO₂ may also allow theSCR device 28 to perform NOx reduction faster and more efficiently asdescribed above. After exiting the oxidation device 22, the firstportion of the exhaust may be directed to the particulate filter 24where particulate matter may be removed.

According to the embodiment shown in FIG. 4B, the first portion of theexhaust may flow through the first leg 30 a″, where the first portion ofthe exhaust may be directed to the partially-loaded oxidation device 22and then the catalyzed particulate filter 32. The partially-loadedoxidation device 22 may have approximately 50% or a lower percentage ofthe holes 22 a coated with platinum. Accordingly, the oxidation device22 may convert some of the NO to NO₂ in the first portion of theexhaust. The increase in NO₂ may allow the regeneration of the catalyzedparticulate filter 32 and may allow NOx reduction by the SCR device 28to be performed faster and more efficiently as described above. Inaddition, the catalyzed particulate filter 32 may allow a substantialamount or all of the first portion of the exhaust to contact theplatinum coating of the filter 32, thereby converting some of the NO toNO₂. The increase in NO₂ may allow the catalyzed particulate filter 32to regenerate as described above and may allow NOx reduction by the SCRdevice 28 to be performed faster and more efficiently as describedabove.

According to the embodiment shown in FIG. 4C, the first portion of theexhaust may flow through the first leg 30 a′″, where reductant may beinjected by the upstream injector 21. Then, the flow of reductant andthe first portion of the exhaust may be directed to the catalyzedparticulate filter 32. The catalyzed particulate filter 32 may allow asubstantial amount or all of the first portion of the exhaust to contactthe platinum coating of the filter 32, thereby converting some of the NOto NO₂. The increase in NO₂ may allow the catalyzed particulate filter32 to regenerate as described above and may allow NOx reduction by theSCR device 28 to be performed faster and more efficiently as describedabove.

According to an exemplary embodiment, the controller 12 may send asignal to the upstream injector 21 inject reductant when a predeterminedcondition is satisfied, e.g., during cold start conditions and/or when atemperature measured by the sensor 40 monitoring the temperature of thecatalyzed particulate filter 32 is within a predetermined range (e.g.,approximately 200° C. to 300° C., approximately 200° C. to 350° C., orother temperature range in which NOx reduction in the SCR device 28 isdependent on the ratio of NO:NO₂). When the predetermined condition issatisfied, the controller 12 may send a signal to the upstream injector21 to inject reductant into the first portion of the exhaust upstreamfrom the catalyzed particulate filter 32. The addition of reductant mayreduce the amount of NOx in the first portion of the exhaust thatcontacts the platinum coating in the catalyzed particulate filter 32.

The controller 12 may be configured to determine a desired (or target)amount of reductant to inject with the upstream injector 21 based on oneor more mappings correlating the sensed temperature of the catalyzedparticulate filter 32 and the desired amount of reductant to inject. Inaddition, the mapping(s) may determine the desired amount of reductantto inject based on a desired amount of NO₂ to supply to the SCR device28, e.g., to reach a desired NO:NO₂ ratio of 50:50. The controller 12may also determine the amount of reductant based on other factors, e.g.,characteristics of the catalyzed particulate filter 32, an estimatedamount of NO₂ in the second portion of the exhaust from the second leg30 b, etc. Accordingly, by using the mapping(s), the controller 12 mayadjust the amount of reductant to inject into the first leg 30 a′″ basedon a sensed condition (e.g., the temperature of the catalyzedparticulate filter 32) to provide more accurate control of the amount ofNO₂ supplied to the SCR device 28. Closed loop control of the ratio ofNO:NO₂ in the first portion of the exhaust may be achieved. Thedetermination of whether and how much reductant to inject with theupstream injector 21 is adjusted based on the sensed temperature of thecatalyzed particulate filter 32.

According to the embodiment shown in FIG. 4D, the first portion of theexhaust may flow through the first leg 30 a″″, where reductant may beinjected by the upstream injector 21. Then, the flow of reductant andthe first portion of the exhaust may be directed to the upstream SCRdevice 23 to reduce the amount of NOx in the flow of exhaust beforebeing directed to the catalyzed particulate filter 32. The catalyzedparticulate filter 32 may allow a substantial amount or all of the firstportion of the exhaust to contact the platinum coating of the filter 32,thereby converting some of the NO to NO₂. The increase in NO₂ may allowthe catalyzed particulate filter 32 to regenerate as described above andmay allow NOx reduction by the SCR device 28 to be performed faster andmore efficiently as described above.

The controller 12 may send a signal to the upstream injector 21 toinject reductant into the first portion of the exhaust when apredetermined condition is satisfied, as described above in connectionwith the embodiment shown in FIG. 4C. The addition of reductant upstreamfrom the upstream SCR device 23 may reduce the amount of NOx in thefirst portion of the exhaust. Since there is less NOx (which includes NOand NO₂), there is proportionally less NO and NO₂ supplied to thecatalyzed particulate filter 32. Therefore, the catalyzed particulatefilter 32 outputs less NO₂ than without the addition of reductant.

The controller 12 may also be configured to adjust the amount ofreductant injected by the upstream injector 21, as described above inconnection with the embodiment shown in FIG. 4C. Accordingly, by usingthe mapping(s), the controller 12 may adjust the amount of reductant toinject into the first leg 30 a″″ based on a sensed condition (e.g., thetemperature of the catalyzed particulate filter 32) to provide moreaccurate control of the amount of NO₂ supplied to the SCR device 28.Closed loop control of the ratio of NO:NO₂ in the first portion of theexhaust may be achieved. The determination of whether and how muchreductant to inject with the upstream injector 21 is adjusted based onthe sensed temperature of the catalyzed particulate filter 32.

According to the embodiment of the exhaust treatment system 50 shown inFIG. 5, the flow of exhaust from the engine 10 may be heated by the heatsource 25 before being directed to the first and second legs 50 a, 50 b.A first portion of the exhaust flows through the first leg 50 a, whereall or a substantial amount of the first portion of the exhaust contactsthe platinum coating of the catalyzed particulate filter 32 to convertsome of the NO to NO₂. The increase in NO₂ may allow the catalyzedparticulate filter 32 to regenerate as described above and may providean increased amount of NO₂ from the first leg 50 a to the SCR device 28.A second portion of the exhaust flows through the second leg 50 b, wherethe second portion of the exhaust is directed to the particulate filter24. The particulate filter 24 may remove particulate matter from thesecond portion of the exhaust. Then, the respective portions of theexhaust from the first and second legs 50 a, 50 b are combined, andreductant is injected by the injector 26 into the combined flow. Thecombined flow is directed to the SCR device 28, which reduces the amountof NOx in the combined flow.

The valves 34 in the first and second legs 50 a, 50 b may be controlledby the controller 12 to control the respective amounts of flow throughthe legs 50 a, 50 b. In an exemplary embodiment, the controller 12 maycontrol the respective amounts of flow through the valves 34 to providea NO:NO₂ ratio that is approximately 50:50 in the flow of exhaustdirected to the SCR device 28. As a result, the reduction of NOx in theSCR device 28 may be more efficient and faster. The exhaust treatmentsystem 50 may reduce particulate matter (with the catalyzed particulatefilter 32 in the first leg 50 a and the particulate filter 24 in thesecond leg 50 b), and may provide a greater reduction in NOx beforereleasing the flow of exhaust to the surrounding atmosphere.

In addition, the controller 12 may control the valves 34 such that thevalves 34 in the first and second legs 50 a, 50 b may be closedsimultaneously. When the valves 34 are closed simultaneously, backpressure is created in the engine 10, which raises the temperature ofthe flow of exhaust. The higher temperature exhaust may be used toregenerate the catalyzed particulate filter 32 in the first leg 50 a andthe particulate filter 24 in the second leg 50 b. Accordingly, the heatsource 25 may be omitted.

According to the embodiment of the exhaust treatment system 60 shown inFIG. 6, the flow of exhaust from the engine 10 may be heated by the heatsource 25 before being directed to the partially-loaded upstreamoxidation device 62. In an exemplary embodiment, the upstream oxidationdevice 62 has a percentage less than 50%, e.g., approximately 25%, ofthe holes 22 a coated with platinum. Alternatively, the upstreamoxidation device 62 may have approximately 50% or another percentageless than 100% of the holes 22 a coated with platinum. Accordingly, theoxidation device 22 may convert some of the NO to NO₂ in the secondportion of the exhaust. Then, reductant may be injected into the flow ofexhaust by the upstream injector 21 and the flow of reductant andexhaust may be directed to the upstream SCR device 23 to reduce theamount of NOx in the flow of exhaust. Since the upstream oxidationdevice 62 converts some of the NO to NO₂ upstream from the upstream SCRdevice 23, greater NOx reduction efficiency may be achieved with theupstream SCR device 23.

The flow of exhaust is then directed to the partially-loaded oxidationdevice 22. In an exemplary embodiment, the oxidation device 22 has apercentage equal to or greater than 50%, e.g., approximately 65%, of theholes 22 a coated with platinum. Alternatively, the oxidation device 22may have approximately 75% or another percentage less than 100% of theholes 22 a coated with platinum. Accordingly, the oxidation device 22may convert some of the NO to NO₂ in the flow of exhaust. The oxidationdevice 22 may convert more NO to NO₂ than the upstream oxidation device62 since the oxidation device 22 has a greater percentage of the holes22 a coated with platinum. Then, optionally, the flow of exhaust may bedirected to the particulate filter 24 where particulate matter may beremoved. The increase in NO₂ by the oxidation device 22 may allow theregeneration of the particulate filter 24, as described above inconnection with the oxidation device 22 and the particulate filter 24 ofthe embodiment shown in FIG. 1.

After exiting the particulate filter 24, reductant is injected into theflow of exhaust by the injector 26, and the flow of exhaust is directedto the SCR device 28, which reduces the amount of NOx in the flow ofexhaust. The remaining NO₂ in the flow of exhaust may be used by the SCRdevice 28 to reduce the amount of NOx with greater efficiency. In anexemplary embodiment, the components of the exhaust treatment system 60,e.g., the upstream oxidation device 62, the upstream SCR device 23, theoxidation device 22, and the particulate filter 24, may be configured toprovide a NO:NO₂ ratio in the flow of exhaust to the SCR device 28 ofapproximately 50:50.

Alternatively or in addition, to provide a NO:NO₂ ratio in the flow ofexhaust to the SCR device 28 of approximately 50:50 or other optimumratio, the sensor 40 may be provided, e.g., downstream of the oxidationdevice 22. The sensor 40 allows the controller 12 to determine theNO:NO₂ ratio and to provide closed loop control of the dosing of thereductant to the upstream SCR device 23 by the upstream injector 21,thereby allowing the conversion efficiency of the upstream SCR device 23to be controlled. The sensor 40 may transmit a signal to the controller12 indicating the NO:NO₂ ratio in the flow of exhaust. For example, thesensor 40 may include the virtual sensor described above and one or morephysical sensors. Then, the controller 12 may determine the timing andamount of reductant injected by the upstream injector 21 to control theNO:NO₂ ratio of the flow of exhaust supplied to the SCR device 28, e.g.,by setting the NO:NO₂ ratio closer to 50:50. For example, one or moremappings may stored in the memory of the controller 12. The mappings maybe used to determine a sensed NO:NO₂ ratio of the flow of exhaust basedon the characteristics sensed by the physical sensors, e.g., thetemperature of the flow of exhaust, space velocity, air flow to theengine 10, etc. Then, the mappings may be used to determine the timingand amount of reductant to inject based on the sensed NO:NO₂ ratio inthe flow of exhaust in order to maintain the NO:NO₂ ratio near 50:50.Thus, closed loop feedback control of the ratio of NO:NO₂ in the flow ofexhaust may be achieved. As a result, the reduction of NOx in the SCRdevice 28 may be more efficient and faster over a wider range ofoperating conditions (e.g., exhaust temperatures). Furthermore, theexhaust treatment system 60 may reduce particulate matter in the flow ofexhaust with the particulate filter 24 and may provide a greaterreduction in NOx before releasing the flow of exhaust to the surroundingatmosphere. Also, as the components of the exhaust treatment system 60age, the closed loop feedback control on the dosing of reductant to theupstream SCR device 23 can compensate for the change in conversionefficiency of the other components, such as the oxidation devices 62,22, thereby maintaining an optimum NO:NO₂ ratio to the SCR device 28.The closed loop feedback control also allows the NO:NO₂ ratio to the SCRdevice 28 to be actively controlled.

Some ammonia may remain in the flow of exhaust after passing through theupstream SCR device 23, e.g., in situations when there is an overdose ofreductant by the injectors 21, 26 or when the oxidation device 22 isunable to convert all of the ammonia. The SCR device 28 may remove theremaining ammonia (i.e., the ammonia slip) by reacting the ammonia withthe NOx in the flow of exhaust to form N₂ and water. Furthermore, thesensor 40 may also be used to determine whether the amount of reductantinjected by the upstream injector 21 is not within a desired range,e.g., too high or too low, so that ammonia slip may be reduced.

According to the embodiment of the exhaust treatment system 70 shown inFIG. 7, the flow of exhaust from the engine 10 may be heated by the heatsource 25 before being directed to the first and second legs 70 a, 70 b.A first portion of the exhaust flows through the first leg 70 a, where apercentage of the first portion of the exhaust contacts the platinumcoating in the upstream oxidation device 62 in the first leg 70 a toconvert some NO to NO₂ The percentage is determined based on the numberof the holes 22 a of the upstream oxidation device 62 that are coatedwith platinum. At the same time, a second portion of the exhaust flowsthrough the second leg 70 b, where a percentage of the second portion ofthe exhaust contacts the platinum coating in the upstream oxidationdevice 72 in the second leg 70 b to convert NO to NO₂. The percentage isdetermined based on the number of the holes 22 a of the upstreamoxidation device 72 that are coated with platinum. Accordingly, anincreased amount of NO₂ may be provided in the first and second legs 70a, 70 b by the respective upstream oxidation devices 62, 72.

Downstream from the upstream oxidation devices 62, 72 in the respectivefirst and second legs 70 a, 70 b, reductant may be injected into theflow of exhaust by the respective upstream injectors 21. The flow ofreductant and exhaust may be directed to the respective upstream SCRdevices 23 to reduce the amount of NOx in the flow of exhaust. Theincrease in NO₂ by the upstream oxidation devices 62, 72 may allow theupstream SCR devices 23 to perform NOx reduction faster and moreefficiently as described above. The respective flows of exhaust are thendirected to the particulate filters 24 in the respective first andsecond legs 70 a, 70 b, and the particulate filters 24 may removeparticulate matter from the respective flows of exhaust. In addition,some of the NO₂ introduced by the upstream oxidation devices 62, 72 intothe flows of exhaust in the respective first and second legs 70 a, 70 bmay be used by the particulate filter 24 for regeneration, as describedabove in connection with the oxidation device 22 and the particulatefilter 24 of the embodiment shown in FIG. 1. The respective portions ofexhaust from the first and second legs 70 a, 70 b are combined anddirected to the SCR device 28, which reduces the amount of NOx in thecombined flow.

The upstream oxidation devices 62, 72 may include different percentagesof holes 22 a coated with platinum. In an exemplary embodiment, apercentage less than 50%, e.g., 25%, of the holes 22 a of the upstreamoxidation device 62 in the first leg 70 a may be coated with platinum,and a percentage greater than 50%, e.g., 65%, of the holes 22 a of theupstream oxidation device 72 in the second leg 70 b may be coated withplatinum. Alternatively, the upstream oxidation device 72 may haveapproximately 75% or another percentage less than 100% of the holes 22 acoated with platinum. The controller 12 may send signals to the upstreaminjectors 21 to control the respective amounts of reductant injectedinto the respective first and second legs 70 a, 70 b, e.g., to provide aNO:NO₂ ratio of approximately 50:50, thereby providing more efficientand faster NOx reduction in the SCR device 28. As a result, the exhausttreatment system 50 may reduce particulate matter (with the particulatefilters 24 in the first and second legs 70 a, 70 b), and may provide agreater reduction in NOx before releasing the flow of exhaust to thesurrounding atmosphere.

In a dual-leg exhaust treatment system, such as the exhaust treatmentsystems 30, 50, 70, 80 shown in FIGS. 3, 5, 7, and 8, the particulatefilter 24 and/or the catalyzed particulate filter 32 in one or both ofthe legs may be a flow through type filter, as described above. Comparedto wall flow type filters, flow through type filters have less backpressure. When back pressure builds up upstream from the flow throughtype filter, there may be a difference between the target or desiredamount of flow through the respective leg and the actual amount of flowthrough the leg. The difference in target/desired and actual amounts offlow may cause a deviation from the target NO:NO₂ ratio, e.g., 50:50,and the actual NO:NO₂ ratio to the SCR device 28. Therefore, when theparticulate filter 24 and/or the catalyzed particulate filter 32 in oneor both of the legs is a flow through type filter, the risk of havingexcessive back pressure is lower, thereby providing a more efficientexhaust treatment system.

One or more characteristics of the exhaust treatment system 20, 30, 50,60, 70, 80, e.g., the percentage of holes 22 a coated in the oxidizingdevice 22, 62, 72, an allocation of flow between two or more legs 30 a,30 a′, 30 a″, 30 a′″, 30 a″″, 30 b, 50 a, 50 b, 70 a, 70 b, 80 a, 80 a′,80 a″, 80 b, 80 b′, etc., may be determined experimentally based on theapplication. For example, the characteristic(s) may be determined whenone or more components of the exhaust treatment system 20, 30, 50, 60,70, 80, e.g., the components of the exhaust treatment system 20, 30, 50,60, 70, 80 upstream from the SCR device 28, are operating at apredetermined operating condition and when the flow of exhaust directedto the SCR device 28 achieves the target NO:NO₂ ratio of 50:50. Thepredetermined operating condition may include, e.g., a predeterminedmass flow, a predetermined temperature, or other operating conditionwhen NOx conversion is difficult and/or when there is less than 50% NO₂.The predetermined mass flow may be, e.g., approximately 60,000 volumehour space velocity (VHSV) (60,000 hr⁻¹) or less, the predeterminedtemperature of the flow of exhaust may be, e.g., approximately 250 to350° C. For example, in one embodiment, the percentage of holes coatedin the oxidizing device and/or the allocation of flow between two ormore legs, may be determined when the following operating conditions areachieved: the components of the exhaust treatment system upstream fromthe SCR device 28 have a VHSV of approximately 60,000 hr⁻¹, thetemperature of the flow of exhaust directed to the SCR device 28 isbetween 250 to 350° C., and the flow of exhaust directed to the SCRdevice 28 achieves a NO:NO₂ ratio of 50:50. Then, the determinedcharacteristic (e.g., percentage of holes coated and/or allocation offlow) may become the target characteristic for the exhaust treatmentsystem 20, 30, 50, 60, 70, 80.

In view of the foregoing disclosure, one skilled in the art may readilyconceive or identify additional configurations of the exhaust treatmentsystem sufficient to realize the desired NO₂ control functions. Forexample, the embodiments of the exhaust treatment systems 80 shown inFIGS. 8 and 9A-9C illustrate additional configurations of thecomponents, e.g., the upstream injector 21, the oxidation device 22, theupstream SCR device 23, and the particulate filter 24, of the exhausttreatment systems shown in FIGS. 1, 3, 4A-4D, and 5-7. The exhausttreatment systems 80 shown in FIGS. 8 and 9A-9C operate using the sameprinciples of operation as described above in connection with FIGS. 1,3, 4A-4D, and 5-7.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the exhaust treatmentsystem. Other embodiments will be apparent to those skilled in the artfrom consideration of the specification and practice of the disclosedexhaust treatment system. It is intended that the specification andexamples be considered as exemplary only, with a true scope beingindicated by the following claims and their equivalents.

1. An exhaust treatment system comprising: an oxidation device having aplurality of channels through which a flow of exhaust flows, a firstportion of the channels being coated with catalytic material forconverting NO to NO₂, a second portion of the channels being uncoatedwith the catalytic material; and a selective catalytic reduction devicedisposed downstream from the oxidation device, the selective catalyticreduction device being configured to receive the flow of exhaust passingthrough the oxidation device.
 2. The exhaust treatment system of claim1, wherein a percentage of the channels of the oxidation device that arecoated with the catalytic material is based on a target NO:NO₂ ratio inthe flow of exhaust supplied to the selective catalytic reductiondevice.
 3. The exhaust treatment system of claim 2, wherein the targetNO:NO₂ ratio in the flow of exhaust supplied to the selective catalyticreduction device is approximately 50:50.
 4. The exhaust treatment systemof claim 2, wherein the percentage of channels coated in the oxidationdevice is determined when operating at a predetermined operatingcondition when the target NO:NO₂ ratio is achieved, the predeterminedoperating condition including at least one of a predetermined mass flowand a predetermined temperature.
 5. The exhaust treatment system ofclaim 1, wherein the first portion of the channels of the oxidationdevice that are coated with the catalytic material constituteapproximately 75% of the channels of oxidation device.
 6. The exhausttreatment system of claim 1, further including a particulate filterdownstream from the oxidation device.
 7. The exhaust treatment system ofclaim 1, wherein the selective catalytic reduction device is adownstream selective catalytic reduction device, and the exhausttreatment system further includes: an upstream selective catalyticreduction device disposed upstream from the oxidation device; and anupstream injector disposed upstream from the upstream selectivecatalytic reduction device, the upstream injector configured to injectreductant into the flow of exhaust.
 8. The exhaust treatment system ofclaim 7, wherein the oxidation device is a downstream oxidation device,and the exhaust treatment system further includes: an upstream oxidationdevice disposed upstream from the upstream selective catalytic reductiondevice, the upstream oxidation device having a plurality of channelsthrough which the flow of exhaust flows, and a percentage less than 100%of the channels of the upstream oxidation catalyst are coated with thecatalytic material.
 9. The exhaust treatment system of claim 8, furtherincluding: a sensor for sensing a characteristic of the flow of exhaustupstream from the downstream selective catalytic reduction device anddownstream from the upstream selective catalytic reduction device; and acontroller connected to the sensor, the controller being configured toreceive the sensed characteristic and control an injection of reductantby the upstream injector based on the sensed characteristic.
 10. Theexhaust treatment system of claim 9, wherein the characteristic is aNO:NO₂ ratio.
 11. The exhaust treatment system of claim 8, wherein thepercentage of coated channels in the downstream oxidation device isgreater than 50%, and the percentage of coated channels in the upstreamoxidation device is less than 50%.
 12. The exhaust treatment system ofclaim 1, further including a first passageway configured to direct aportion of the flow of exhaust around the oxidation device, theoxidation device being disposed in a second passageway.
 13. The exhausttreatment system of claim 12, further including: a first particulatefilter disposed in the first passageway; and a second particulate filterdisposed in the second passageway.
 14. The exhaust treatment system ofclaim 13, wherein at least one of the particulate filters is coated withthe catalytic material for converting NO to NO₂.
 15. The exhausttreatment system of claim 13, wherein at least one of the particulatefilters is a flow through type filter.
 16. The exhaust treatment systemof claim 12, wherein the oxidation device is a second oxidation device,and the exhaust treatment system further includes a first oxidationdevice in the first passageway, the first oxidation device having aplurality of channels through which the flow of exhaust flows, apercentage less than 100% of the channels of the first oxidation devicebeing coated with the catalytic material.
 17. The exhaust treatmentsystem of claim 16, wherein: the selective catalytic reduction device isa downstream selective catalytic reduction device disposed downstreamfrom the first and second passageways; and the exhaust treatment systemfurther includes: a first selective catalytic reduction device in thefirst passageway downstream from the first oxidation device; and asecond selective catalytic reduction device in the second passagewaydownstream from the second oxidation device.
 18. The exhaust treatmentsystem of claim 17, wherein the percentage of channels coated with thecatalytic material in the first oxidation device is greater than 50%,and the percentage of channels coated with the catalytic material in thesecond oxidation device is less than 50%.
 19. The exhaust treatmentsystem of claim 17, further including: a first particulate filter in thefirst passageway downstream from the first selective catalytic reductiondevice; and a second particulate filter in the second passagewaydownstream from the second selective catalytic reduction device.
 20. Theexhaust treatment system of claim 1, wherein the catalytic material isplatinum.
 21. A method for treating a flow of exhaust comprising:generating the flow of exhaust; passing the flow of exhaust through aplurality of channels of a downstream oxidation device, a first portionof the channels being coated with catalytic material for converting NOto NO₂, a second portion of the channels being uncoated with thecatalytic material; and directing the combined flow of exhaust to adownstream selective catalytic reduction device disposed downstream fromthe downstream oxidation device.
 22. The method of claim 21, furtherincluding: passing the flow of exhaust through a plurality of channelsof an upstream oxidation device disposed upstream from the downstreamoxidation device, a first portion of the channels of the upstreamoxidation device being coated with the catalytic material, a secondportion of the channels of the channels of the upstream oxidation devicebeing uncoated with the catalytic material; injecting reductant with anupstream injector disposed downstream from the upstream oxidation deviceand upstream from the downstream oxidation device; and passing the flowof exhaust through an upstream selective catalytic reduction devicedisposed upstream from the downstream oxidation device and downstreamfrom the upstream injector.
 23. The method of claim 22, furtherincluding: sensing a characteristic of the flow of exhaust; andcontrolling the injection of reductant by the upstream injector based onthe sensed characteristic.
 24. The method of claim 23, wherein thesensed characteristic is a sensed NO:NO₂ ratio in the flow of exhaust.25. The method of claim 24, wherein the injection of reductant by theupstream injector is controlled based on a target NO:NO₂ ratio in theflow of exhaust supplied to the downstream selective catalytic reductiondevice.
 26. The method of claim 25, wherein the target NO:NO₂ ratio inthe flow of exhaust supplied to the downstream selective catalyticreduction device is approximately 50:50.
 27. An exhaust treatment systemcomprising: an upstream oxidation device having a plurality of channelsthrough which a flow of exhaust flows, a first portion of the channelsbeing coated with catalytic material for converting NO to NO₂, a secondportion of the channels being uncoated with the catalytic material; anupstream injector configured to inject reductant into the flow ofexhaust upstream from the upstream selective catalytic reduction device;an upstream selective catalytic reduction device disposed downstreamfrom the upstream injector, the upstream selective catalytic reductiondevice being configured to receive the flow of exhaust; a downstreamoxidation device disposed downstream from the upstream selectivecatalytic reduction device, the downstream oxidation device having aplurality of channels through which the flow of exhaust flows, a firstportion of the channels of the downstream oxidation device being coatedwith the catalytic material, a second portion of the channels of thedownstream oxidation device being uncoated with the catalytic material;and a downstream selective catalytic reduction device disposeddownstream from the downstream oxidation device, the downstreamselective catalytic reduction device being configured to receive theflow of exhaust.
 28. The exhaust treatment system of claim 27, furtherincluding: a sensor configured to sense a characteristic of the flow ofexhaust; and a controller connected to the sensor, the controller beingconfigured to receive the sensed characteristic and control an injectionof reductant by the upstream injector based on the sensedcharacteristic.
 29. The exhaust treatment system of claim 28, whereinthe sensed characteristic is a sensed NO:NO₂ ratio in the flow ofexhaust.
 30. The exhaust treatment system of claim 29, wherein thecontroller controls the injection of reductant based on a target NO:NO₂ratio in the flow of exhaust supplied to the downstream selectivecatalytic reduction device.
 31. The exhaust treatment system of claim30, wherein the target NO:NO₂ ratio in the flow of exhaust supplied tothe downstream selective catalytic reduction device is approximately50:50.